This invention relates to an internal shoe drum brake, primarily for a motor vehicle, and of the general kind having a pair of brake shoes mounted on a backplate and separable by an actuator into braking engagement with a rotary brake drum, one pair of adjacent shoe ends providing shoe tips slidably engaged respectively with oppositely directed faces of a fixed abutment which resists braking torque applied to the shoes from the rotating drum during braking.
In two-leading shoe or Duplex drum brakes, the shoes have generally similar operating characteristics and their lining thickness and arcuate extent, as well as their abutment shoe tips can be identical. The shoes are therefore interchangeable, leading to considerable cost savings in terms of reduced parts inventory and simplified production. Such interchangeability of the shoes also provides safety benefit in that the risk of incorrect assembly is considerably reduced.
The operating characteristics of some leading/trailing shoe or Simplex brakes require different specific orientations of the respective abutment shoe tips and the abutment surfaces on which they are supported in order to achieve the desired performance from this type of brake. The leading shoe in some such brakes tends to wear the more rapidly and is traditionally provided with a thicker lining than the trailing shoe. Such a brake is therefore inherently expensive to produce and carries an attendant risk of incorrect assembly of the brake shoes. Standardising the thickness and arcuate extent of the shoe lining in such a brake requires the angles of the abutment surfaces to differ substantially from each other and positioning of the shoe tip contact points at specific locations on the abutment is also necessary. Such requirements can be incompatible with the brake geometry of existing brakes and unsatisfactory performance can result.